The invention relates to a cinematograph camera flicker control and/or indicating device. It also relates to a cinematograph camera including such a device.
It is well known that the cinematographic process consists of taking a series of individual photographic exposures, usually at 24 or 25 frames per second. For the image density of successive frames to be consistent the photographic exposure must be the same for each frame. If the density of successive images is not substantially the same and if the changes are significant and rapid the picture will appear to flicker on projection. If, on the other hand, the changes in exposure are slow the projected picture will alternate between light and dark.
No problem generally arises when filming under natural or under incandescent light, since the intensity is generally fairly constant. Although incandescent lamps energised by an AC power supply do flicker slightly, at twice the supply frequency, the intensity variation is so small (no more than about 3%) that it does not noticeably affect the photographic image density.
A problem occurs, however, when AC powered short-arc metal halide, fluorescent and other non-incandescent light sources are used, where the difference in light intensity between the brightest peaks and dimmest troughs may be as much as 70%, or more. In this case, if successive images are exposed during different periods of the lighting cycle they will have considerably different densities, and the resulting picture will therefore appear to flicker when projected.
Between these two extreme conditions there is a "grey area" where there is flicker present but where the effect is within acceptable limits.
To eliminate any possibility of uneven exposure between successive images it is necessary to ensure that either the camera speed, or the exposure period are compatible with the frequency of the AC source powering the lighting. It will be appreciated, of course, that since the exposure period is simply a product of the camera speed (in frames per second) and the angle of the open segment or segments of the rotating or oscillating camera shutter, the camera speed combined with the shutter angle rather than the exposure period could be taken as one of the variables.
Compatibility, that is the substantial absence of flicker, requires one of the following:
1. The camera speed, in frames per second, is substantially exactly divisible into the number of light brightness periods per second. For example, this may be achieved by filming at 25 frames per second when the subject is illuminated by lamps powered by 50 Hz AC (100 peak brightness periods per second) or by filming at 24 frames per second with a 60 Hz power supply (120 peak brightness periods per second). In this case the duration of the exposure period is not important.
2. The duration of the exposure period is substantially exactly the same length as an integral number of periods of lamp brightness. In this case, the camera speed is not important since the integrated exposure over the whole exposure period will always be that produced by a complete number of cycles, no matter where in the cycle the shutter opens.
To achieve these conditions it has been common practice manually to set the shutter opening angle to suit the chosen camera speed and power frequency; or to set the power frequency to suit the chosen camera speed and shutter opening angle; or to set the camera speed to suit the chosen power frequency or combination of power frequency and shutter opening angle. One of the disadvantages of this manual method is that it is very inflexible; any fluctuation from the optimum by any one of the three variables may result in an uneven exposure, and therefore apparent flicker on projection. It is also difficult to make quick and accurate changes manually to any one or combination of the variables while filming; the result of this is that it is impossible to use an in-shot variable shutter facility when filming under non-incandescent artificial light sources. Such variable shutter facilities, and variable camera speed facilities are available on commercial cameras and it would be desirable if the camera could be used to its fullest extent over a wider range of lighting conditions than at present.
A further disadvantage with the prior art is that the camera operator has no indication, when filming, of whether the lighting conditions and the exposure period/camera speed combination he is using is likely to produce flicker when the resulting images are projected, or, if flicker is present, how bad it is likely to be. He has no idea, for example, whether the operating conditions he is using mean that the flicker will fall within acceptable tolerance limits. For this reason, the camera operator has to restrict himself to operating conditions where he is certain that the flicker will be small. He is therefore not able to be flexible in his use of camera speeds and shutter openings. It is also difficult to film under lighting powered by a mobile alternator which does not run at an exact and constant speed, since an operator filming under such lighting is forced to use camera speeds and/or exposure periods which correspond to settings where the tolerance of lighting fluctuations is large.